A SAW (Surface Acoustic Wave) device has been known as one of the devices that uses an acoustic wave. The SAW device has been used in a variety of circuits that process radio signals in a frequency band of 45 MHz to 2 GHz typified by mobile phones, for example. Examples of the variety of circuits include a transmission band-pass filter, a reception band-pass filter, a local filter, an antenna duplexer, an IF filter and an FM modulator.
As the performance of mobile phones has been enhanced and the sizes of mobile phones have been also reduced in recent years, an improvement in a variety of properties, e.g., an increase in out-of-band suppression and an improvement in temperature stability, and a reduction in device size have been demanded. As a way to improve the temperature stability, a technique of forming a dielectric material, such as SiO2, on an interdigital transducer on a piezoelectric substrate has been developed. Furthermore, a boundary acoustic wave device or the like has been developed. In the boundary acoustic device, a different dielectric material having a higher acoustic velocity is formed on the dielectric material, so that energy is trapped in the boundary between the dielectric materials and the surface of the piezoelectric substrate. Thereby, the device size has been reduced.
Frequency variations due to manufacturing variations could be a common problem to all of these acoustic wave devices. As measures against this problem, a variety of methods for making a frequency adjustment have been disclosed (see Patent documents 1 and 2, for example).
As one example, it has been proposed to make a frequency adjustment by forming a SiN film on an interdigital transducer, a reflector and a piezoelectric substrate using plasma chemical vapor deposition or the like (see Patent document 1, for example). Further, it has been proposed to make a frequency adjustment by forming a SiN layer on a SiO2 layer and physically etching the SiN layer to reduce its thickness or spattering the SiN layer to increase its thickness (see Patent document 2, for example).
Further, it has been proposed to adjust the frequency of a boundary acoustic wave device including an IDT electrode between a first medium and a second medium by changing the thickness of the second medium (see Patent document 3, for example).    Patent document 1; Japanese Laid-open Patent Application No. H2-301210    Patent document 2: WO 2005/083881    Patent document 3: WO 2005/093949
As described above, as a method of making a frequency adjustment, a frequency characteristic is adjusted by etching the medium covering the electrode in the depth direction to adjust the thickness of the medium. For example, in order to bring variations in frequency characteristic between a plurality of acoustic wave devices formed on a single wafer into a desired range by using this method, it is necessary to select a portion of each acoustic wave device on the wafer and adjust the thickness of the medium on the selected portion. For example, the thickness of the medium needs to be adjusted by masking a portion of the wafer and etching a portion of the upper surface of the medium in the depth direction. In this way, an etching amount varies depending on each chip.
In this case, in order to bring the frequency distribution in the wafer surface into a desired range, masking is carried out for each chip, and even in some cases, film formation and etching need to be repeated several times. As a result, problems, such as an increase in the number of processes, arise.